This invention relates to improving the properties of minerals rich in carbon such as coal, shale oil, crude oil, peat and fractions thereof, including particularly petroleum gas oils, specifically heavy vacuum gas oils. It more particularly refers to a novel method of reducing the ash content, increasing the specific caloric value, that is the number of calories available per unit of weight, and improving the fluidity of such carbonaceous materials. It still further refers to a novel technique for reducing the oxygen, and if present the metal, content of such carbonaceous materials.